The present invention relates to a method of generating and detecting acoustic surface waves in a material, which method is particularly useful in the non-destructive testing of the material by measuring the velocity of an acoustic surface wave therethrough. The invention also relates to apparatus, and particularly to a novel probe, useful in such method.
Acoustic surface waves are employed as an investigative tool in several aspects of non-destructive testing of materials, particularly metals, which tests include the measurement of surface wave attenuation coefficients, reflection coefficients, and velocity. For example, the onset of fatigue or stress corrosion failure is accompanied by microstructural changes in the material under consideration, and these have been correlated with shifts in the attenuation of acoustic surface waves. In addition, empirical relations have been determined relating attenuation coefficient to grain size in various materials. Surface waves have been used to detect and measure surface-breaking cracks through partial reflection of the propagating waves. Directional dependence of surface wave velocity has been attributed to the presence of preferred grain orientation. It has also been shown that surface wave velocity changes of the order of a fraction of one percent are induced by internal surface stresses, this latter interaction being termed the "acousto-elastic" effect.
Acoustic wave attenuation changes and variations in wave velocity that result from the various effects described above are in most cases small perturbations only. In order to determine meaningful correlations between attenuation or velocity and the various independent variables, detection and monitoring equipment with a high degree of repeatability and resolution is required. In addition, during the course of measurement this instrumentation must not change surface properties of the material being examined.
The most common method, known as the "critical angle" technique, for generating and detecting surface waves on a test sample surface employs plastic wedges and is based on Snell's Law. In this technique a compression wave, generated in the plastic wedge by a transducer, impinges on the test sample surface at the "critical angle" required for the generation of a surface wave therealong. The surface wave may be detected by the reverse process. Oil or some other fluid is usually used as a coupling medium between the plastic wedge and test sample surface to ensure passage of the various waves.
A second known, but less commonly applied, method for generation of surface waves, which may be termed the "driven wedge" technique, is based on the fact that a sharp metal wedge driven vertically into a test sample surface will generate surface waves in two opposing directions along the test sample surface. The driving force may be a compression wave generated by a transducer mounted at the top end of the wedge and travelling in the direction of the sharp edge of the wedge. A travelling surface wave may be detected by an identical wedge in contact with the surface, as the shear component of the wave couples to the wedge, thereby generating a bulk wave which travels in the wedge away from the contact plane and in the direction of the transducer.
Both of the above techniques have been adapted to the development of surface wave devices including generating wedges and similar detecting wedges. However, we have found that these known devices using similar generating and detecting wedges have a number of serious drawbacks which substantially reduce their feasibility, or even preclude their use, in many non-destructive testing applications, particularly for the measurement of surface wave velocity changes as a function of externally applied stresses. For example, it was found that in the "critical angle" technique, the high temperature sensitivity of acoustic velocity in plastic is such that very small temperature fluctuations (of 0.1.degree. C.) create such variations in the acoustic velocity as to substantially mask the variations therein caused by the stresses to be measured. With respect to the "driven wedge" technique, it was found that the generated acoustic wave is not focused, and therefore much of the initial intensity of the waves generated by the generator wedge is lost through reflections from the sides of the wedge as the wave travels towards its narrow edge, thereby producing an output having a low signal-to-noise ratio.
An object of the present invention is to provide an improved method and apparatus for generating and detecting an acoustic surface wave particularly useful for the non-destructive testing of materials, which method and apparatus have advantages in the above respects.